Modern electron microscopes are mostly equipped with a so-called single-field condenser objective lens (SFCO-lens) of Riecke and Ruska as described, for example, in German Patent 875,555. Here, for a suitable incident beam, the symmetrical positioning of the object in the gap center of the pole pieces makes possible the high resolution transmission operation (TEM-operation) and the generation of very small electron probes for the scanning transmission operation (STEM-operation) and the micro diffraction. For TEM-operation, the last condenser lens images the electron source into the front focal point of the SFCO prefield lens and thereby effects an axially parallel illumination of the object. The magnitude of the illumination field of the object is then determined by the diameter of the so-called condenser diaphragm (illumination diaphragm). For STEM-operation, the electron source is imaged by the last condenser lens into the imaging plane of the SFCO prefield lens which images the electron source demagnified into the object.
However, this TEM illumination has the disadvantage that the imaging of the electron source into the front focal point of the SFCO prefield lens must be strictly maintained; otherwise, the short focal length prefield of the SFCO lens effects an extremely slanted illumination of the off-axis object area. The last condenser lens can therefore not be utilized to control the magnitude of the illumination field from the object and its aperture (brightness).
German published patent application 2,882,242 discloses the installation of an auxiliary lens close ahead of the SFCO lens. This auxiliary lens and the SFCO prefield lens conjointly define a telefocal lens system. With this auxiliary lens, an illumination of the object is obtained which corresponds to the known transmission electron microscopes and therefore has the disadvantage that the illuminating aperture and the illumination field cannot be adjusted independently of each other. The auxiliary lens must be switched off or its action minimized for generating small probes in STEM operation whereby the adjustment of the SFCO lens can change.
An independent adjustment of the illuminating aperture and the illumination field of the object is discussed in German Patent 1,614,123. Here, first a greatly reduced image of the electron source (crossover) is generated by a first condenser lens having a short focal length and this image is imaged into the focal plane of the SFCO prefield lens by a second condenser lens having a long focal length. Behind the second condenser lens, the illumination field diaphragm is disposed which is sharply imaged into the object plane by the SFCO prefield lens thereby determining the magnitude of the illuminated object area. The illuminating aperture is determined by the magnitude of the crossover in the focal plane of the SFCO prefield lens and is changed by the degree of demagnification of the first condenser lens which, for this purpose, has a selectively insertable pole piece system effecting different image distances.
A disadvantage of this solution is that the selectively insertable pole piece systems are mechanically very complex and they require a follow-up adjustment and permit only a slight variation of the illumination aperture in few stages. No information is provided for changing the illumination field diaphragm so that only the known exchanging devices can be considered with the disadvantage of the necessary follow-up adjustment which for routine work by an operator is most annoying and which can hardly be asked of such an operator because of the location of the diaphragms.
An optimal illumination of the object in a transmission electron microscope must fulfill the following requirements:
(a) the illumination aperture (aperture angle of the illuminating beam cone) must be variable in order to be able to adapt the coherence of the illumination, the image brightness and the contrast to the particular object;
(b) the magnitude of the illuminated object area (illumination field) can only be slightly larger than the imaged object area in order to prevent unnecessary damage to the object and stray electrons which reduce contrast;
(c) the adjustment of the illuminating aperture and the magnitude of the illumination field must be independent of each other;
(d) the illumination beam solid angle must be incident on every point of the object as perpendicularly as possible in order to prevent image distortions through inclined illumination in the off-axis region; and,
(e) the illumination field must be uniformly illuminated and without structure.
In the area of light microscopy, these requirements are fulfilled in an illuminating system by the so-called Kohler illumination. This principle has been applied in electron microscopy in only a few cases for individual magnification adjustments notwithstanding its clear advantages. This situation is present because for electron optical systems, there are no diaphragms having a variable diameter and having the required edge definition and for this reason a continuous exchange of the diaphragms would be required. This exchange is however unacceptable because of the position of the diaphragms and the time consumed for exchanging the diaphragms and the required follow-up adjustments after each exchange in routine operation.